1. Field of the Invention
This invention relates to a process and apparatus for the treatment of waste water containing nitrogenous materials and is more particularly concerned with a process and apparatus for the recovery of useful gaseous feedstocks from waste water contaminated with urea and like materials.
2. Description of the Prior Art
Urea is manufactured commercially by reacting ammonia with carbon dioxide. The reaction produces substantial amounts of water (one mole for each mole of urea produced). Illustratively, a 1900 short ton capacity urea plant produces about 50,000 pounds per hour of water in the urea reaction. An additional 20,000 pounds per hour of water is condensed from utility steam in those plants that have steam-motivated vacuum jets and this steam condensate is mixed with the water generated in the reaction. Urea and carbamate are separated from the reaction products leaving waste water (hereafter "process condensate") contaminated with minor amounts of urea and other combined forms of ammonia and carbon dioxide including ammonium carbonate, ammonium carbamate and the like (hereinafter referred to collectively as "urea values"). This process condensate must be treated to reduce the level of urea values before it can be utilized or discarded. If the condensate is not so treated, but is disposed of by other means which results in its introduction into natural waterways, the urea values therein promote algae growth. In addition, ammonia produced by hydrolysis of the urea values can prove toxic to fish in the waterways.
Similarly, if the process condensate is to be re-utilized, a circumstance which is of particular importance where the urea plant complex is located in a geographical area where fresh water is not abundant, the urea values must be reduced to a level at which they no longer cause corrosion problems in processing equipment before the recovered process condensate can be employed as, for example, cooling water, boiler feed water and the like.
Accordingly, it has hitherto been common practice to subject the recovered process condensate to hydrolysis and use a countercurrent stream of steam in a stripping tower to convert the urea values to ammonia and carbon dioxide. The waste water so recovered still contains sufficient contaminates to require further treatment with anti-corrosion agents before re-use in steel processing equipment. A recently described improvement in the hydrolytic treatment process (U.S. Pat. No. 4,341,640) requires the use of a vessel which contains a special liquid holding zone located within the vessel and substantially separated from the vapor streams passing through the vessels.
U.S. Pat. No. 4,264,567 at Col. 5, lines 1-16, makes brief mention of the use of urea process condensate to cool steam employed in a process for producing hydrogen feedstock.
In all the hydrolytic processes previously employed the main object has been to reduce or eliminate, as far as possible, the contaminants in the process condensate so as to render the water useful in the liquid state for other purposes such as cooling water, boiler feed water and the like and to reduce toxicity of the effluent water.
James U.S. Pat. No. 3,292,998 describes a process and apparatus for saturating a feed gas stream of carbon monoxide with water vapour prior to catalytic reaction to produce hydrogen and carbon dioxide. Pinto U.S. Pat. No. 4,072,625 describes a process and apparatus for saturating a hydrocarbon feedstock with water vapor prior to catalytic reforming. The hydrocarbon feedstock is brought into contact under pressure with water which has been heated by scrubbing the synthesis gas stream from the catalytic reaction. A very closely related process is discussed by G. R. James in a paper entitled Synthesis Gas Condensate Stripping presented at the 168th ACS National Meeting, Atlantic City, N.J. on Sept. 11, 1974.
I have now found that urea plant process condensate can be treated on a continuous basis and in a highly economical and efficient manner to convert substantially the whole of the process condensate to a gaseous stream which is especially adapted for use as feedstock for an ammonia plant reformer. The latter forms part of the plant complex utilized in the manufacture of urea.
The advantages in process economics, re-utilization of waste products, reduction in energy requirements and other benefits to be discussed below, which flow from my discovery will be readily apparent to one skilled in the art.